This invention relates to a method of manufacturing a crystallized thin film formed by ferroelectric substance, such as Pb (Zr, Ti) O3 (PZT) by the use of a sol-gel method.
When a ferroelectric thin film is used as a capacitor insulating film of a non-volatile memory, it is indispensable to reduce an area of a memory cell in order to improve the integration of the memory.
To this end, it is necessary to directly form the ferroelectric thin film capacitor on a conductive plug which is recently applied to a DRAM (Dynamic Random Access Memory) having high integration.
In this event, when a thermal treatment is performed during a production of the ferroelectric thin film capacitor, the conductive plug and a diffusion barrier layer attached thereto (for example, TiN/Ti) are oxidized. Consequently, the conductivity is often and inevitably lost.
Therefore, it is required that a temperature during the production of the ferroelectric thin film is reduced to 500xc2x0 C. or less, more preferably 450xc2x0 C. or less, to avoid such oxidation.
Meanwhile, it is well known that Pb base ferroelectric substance, in particular, Pb (Zr, Ti) O3 (hereinafter, abbreviated as PZT) and material added slight of additive such as La and Nb into PZT is suitable as ferroelectric thin film material for the non-volatile memory. This is because the Pb base ferroelectric substance has a large residual polarization, and can be produced at about 600xc2x0 C.
As the production method of the ferroelectric thin film, the sol-gel method is desirable because it has such advantage that an excellent thin film can be obtained with superior repeatability using a cheaper equipment. In such a sol-gel method, an organic metal material is dissolved into desired solvent, and is applied and baked.
For example, it has been reported that PZT of Zr/Ti=53/47 becomes a single perovskite phase by using a buffer layer of PbTiO3 (hereinafter, abbreviated as PT) by the sol-gel method at 500xc2x0 C. written in Journal of material research 1993, Vol. 8. Page 339 (C. K. K wok et al., J. Mater. Res. 8, 339 (1993)).
In this case, the production process of the PZT thin film is illustrated in FIG. 1, and the PT layer is crystallized before applying PZT. In this paper, although a sapphire substrate is used, the electrical characteristic such as ferroelectric characteristic is not reported at all.
Further, disclosure has been made about such a fact that PZT is produced using a PT buffer layer by the sol-gel method at 450xc2x0 C. in Japanese Journal of Appl. Phys, 1996, Vol. 35, page 4896 (H. Suzuki et al., Jpn. J. Appl. Phys. 35, 4896 (1996)).
Although the single perovskite phase is formed 450xc2x0 C. ,as illustrated in FIG. 4 in this paper, the paper does not disclose or teach the ferroelectric characteristic.
Moreover, the dielectric constant is 30 or less at about 0.2 xcexcm, as shown in FIG. 5 in this paper, and the characteristic is not enough to be practically used.
The process for producing the PZT thin film disclosed in this paper is illustrated in FIG. 2, and the PT layer is decomposed by an organic thermal process at 350xc2x0 C. before applying PZT.
As mentioned above, the Pb base ferroelectric substance, in particular, the PZT based ferroelectric thin film (the film thickness of 300 nm or less) having excellent composition at 500xc2x0 C. or less, more desirably 450xc2x0 C. or less has not been realized by the use of the sol-gel method
It is therefore an object of this invention to provide a method of manufacturing a thin film and a capacitor using excellent PZT base ferroelectric material at a low temperature by the use of the sol-gel method.
In a method of manufacturing a thin film according to this invention, a buffer layer is formed on a substrate. Thereafter, a ferroelectric thin film material is applied thereto before thermally decomposing the buffer layer.
Subsequently, the buffer layer and the ferroelectric thin film are decomposed together. Finally, a crystallized thermal process is performed.
In this event, the buffer layer is provided so as to proceed the crystallization of the thin film on the buffer layer, and may be referred to as a seed forming layer.
The deposition temperature due to the sol-gel method can be lowered by forming the thin film using such a method.
When this method is applied to the PZT thin film, a buffer layer containing PbTiO3 as main component is formed on the substrate.
Thereafter, a thin film material containing PZT as main component is applied before decomposing the buffer layer by a thermal organic process.
After the buffer layer and the thin film are decomposed together by the thermal organic process, a crystallized thermal process is performed.
More specifically, after the buffer layer containing PbTiO3 as main component is formed on the substrate, the buffer layer is baked at a temperature at which organic thermal decomposition does not occur.
Subsequently, a thin film material containing PZT as main component is applied on the buffer layer.
After the PZT thin film is baked at a temperature at which the organic thermal decomposition does not occur, the buffer layer and the thin film are decomposed together by the thermal organic process. Finally, the crystallized thermal process is performed.
In this case, the application step of the thin film containing PZT as main component through the crystallized thermal step may be repeated after the crystallized thermal process such that the PZT thin film has the preselected film thickness.
In this event, the duration of the final crystallized thermal process may be longer than that of the previous crystallization thermal process.
This reason will be explained hereinbelow. Namely, when the crystallization is carried out at such a low temperature, as thermal process duration is longer, the characteristic such as the ferroelectric characteristic is more improved.
If the crystallization thermal process is performed for long duration at every application layers, the final crystallization thermal process is unnecessary.
However, long duration is required to manufacture the thin film when the application number is particularly increased. In consequence, the throughput is degraded.
In the meantime, the layer, which is initially applied, is subjected to the thermal process having the longest duration. Consequently, the crystallized thermal process durations are variable for the respective application layers, and the thin film may be formed such that each application layer has not a uniform characteristic.
To this end, it is preferable that the final crystallization thermal process, which is entirely performed, is carried out for longer duration.
Further, in a method of manufacturing a PZT thin film according to this invention, a buffer layer containing PbTiO3 as main component is formed on the substrate.
Thereafter, the buffer layer is baked at a temperature at which the organic thermal decomposition does not occur.
Subsequently, the thin film material containing PZT as main component is applied on the buffer layer.
After the PZT thin film is baked at a temperature at which the organic thermal decomposition does not occur, the buffer layer and the thin film are decomposed together by the thermal organic process. Finally, the crystallized thermal process is entirely performed.
In this case, the application step of the thin film containing PZT as main component through the thermal decomposition step may be repeated after the crystallized thermal process such that the PZT thin film has the preselected film thickness.
Further, RTA (Rapid Thermal Annealing) decomposition may be performed after decomposing by the organic thermal process.
Alternatively, the RTA decomposition may be carried out instead of the organic thermal decomposition.
The general organic thermal decomposition is carried out within the temperature range between 300xc2x0 C. and 400xc2x0 C. for process duration of about 10 minutes in the oxygen atmosphere (in oxygen gas or in H2O/O2 atmosphere).
However, the RTA decomposition is conducted at a slightly higher temperature although the process atmosphere is not changed.
Namely, the RTA is carried out within the temperature range between 430xc2x0 C. and 450xc2x0 C. during several seconds to several minutes, more specifically, for about 15 seconds to about 2 minutes.
In case that the crystallization is carried out after performing the PZT application for several times, carbon in the film is not sufficiently decomposed and may be remained in the film as impurity when the application is performed again after conducting only the general organic thermal decomposition.
Therefore, it is preferable to combine the RTA decomposition after the general organic thermal decomposition.
When the crystallization thermal process temperature exceeds 500xc2x0 C. in the above-mentioned manufacturing method of the PZT thin film, the crystallization of the perovskite phase starts without the PT buffer layer, and the effect of the PT buffer layer is reduced. Therefore, the temperature range between about 430xc2x0 C. and 450xc2x0 C. is desirable.
The thermal process temperature is variable in dependence upon the composition ratio between Zr and Ti in PZT. As the ratio of Ti is higher, the perovskite phase is readily generated at the lower temperature.
As the crystallization thermal process temperature is lower, the pyrochlore phase, which does not represent the ferrroelectric characteristic, is more easily generated. This phenomenon is well known in the art.
However, it is possible to obtain the crystallized phase of the perovskite phase at the temperature of 430xc2x0 C. or higher according to this invention.
Further, the baking process is prepared in addition to the crystallization or the organic thermal decomposition. The baking process is carried out to dry solvent, and can dry the thin film by performing the thermal process in the desired atmosphere without the organic thermal decomposing within the temperature range between about 100xc2x0 C. and 250xc2x0 C. for about 10 minutes although a slight change occurs in accordance with the kind of the solvent.
Moreover, this invention is applicable as a method of manufacturing a capacitor using the PZT thin film. In this case, an upper electrode may be formed after crystallizing the PZT thin film.
Preferably, it is possible to obtain the ferroelectric thin film superior in performance by crystallizing the PT thin film with the PZT thin film together after forming the upper portion electrode.
Further, it is preferable that the film thickness of the buffer layer containing PbTiO3 as main component is thinner taking the performance of the obtained capacitor into account. However, it is permitted that the thickness is 10% or less of the film thickness of the layer containing PZT as main component.
In FIG. 3A, the upper electrode is formed after crystallizing the PZT thin film.
Although the pervskite phase serving as the ferroelectric phase is crystallized at 450xc2x0 C. or less in PT, PT itself is not suitable as the capacitor for the memory because anti-electric field is large and the repeating operation resistance is also small.
However, PT has such a characteristic that the crystallization is carried out at the lower temperature and the PT is similar with PZT in the crystal structure and the lattice constant.
With this characteristic, the energy for crystallizing PZT can be reduced by using it as the buffer layer during the deposition of PZT.
Such an effect results in the following mechanism. Namely, when the laminated PT layer and PZT layer in the non-crystallized state are subjected to the thermal process, the PT layer having small energy for crystallizing is first crystallized, and the PZT layer is successively crystallized.
Such continuous crystallization readily proceeds when the interface between the PT layer and the PZT layer has a certain degree of slope by mutual diffusion.
Therefore, the reduction effect of the PZT crystallization temperature of the PT buffer layer is most effectively obtained when a certain degree of mutual diffusion occurs at the interface between PZT and PT at the stage before crystallization of PZ, and PZT and PT in the non-crystallized state are crystallized together.
To obtain such a state, it is effective to apply the PZT layer before performing the organic thermal decomposition of the PT buffer layer and conducting the organic thermal decomposition for the both together in the sol-gel method. This is because the entire organic thermal deposition process causes the mutual diffusion of the interface of PZT/PT.
The continuous crystallization from the buffer layer side proceeds by performing the crystallized thermal process in this state, and the crystallization of PZT becomes possible at the low temperature between about 430xc2x0 C. and 450xc2x0 C.
In FIG. 3B, the PT thin film and the PZT thin film are crystallized after forming the upper electrode. In this method, it is possible to improve the polarization switching characteristic of the ferroelectric thin film capacitor formed at the low temperature between about 430xc2x0 C. and 450xc2x0 C. at low electric field.
The upper electrode is formed before the crystallized thermal process. Thereby, the stress inside the PZT thin film generated during the crystallization is reduced by clamping the upper interface of the PZT thin film before the crystallization.
Further, defects generated at the interface is reduced by thermally processing the interface between PZT and the upper electrode at the same time as the PZT crystallization.
As a result, the polarization conversion characteristic of the PZT thin film capacitor formed under the low temperature can be improved at the low electric field.